The present invention relates to a circulation type media agitator mill. The circulation type media agitator mill in accordance with this invention is appropriate for grinding or dispersing into powder a variety of small pieces of material such as ink, paint, dying material, ceramic material, metal, inorganic material, ferrite, toner, glass or pigmented paper-coating material. These materials, however, should not be understood as restrictive.
Japanese Patent Publication No. H 02-10699 shows a conventional mill of the type which has being in practical use. It comprises: a mill body which includes a grinding chamber to be filled at least partly with grinding media and material to be ground and has an inlet for material to be ground and an outlet for crushed material, an agitator shaft having an inner shaft end inside the grinding chamber, and a separating means permitting finished pulverized material to flow out of the grinding chamber to the outlet yet retaining grinding media, wherein the agitator shaft has an end portion formed with a cavity therein which is open at the inner shaft end, the end portion of the agitator shaft includes recesses distributed around the cavity and spaced from the inner shaft end, through which said grinding media from said grinding chamber may axially flow into the cavity and through the inner shaft end to flow back into the grinding chamber, and the separating means is arranged at least substantially inside the cavity.
This type of mill is so designed that the material to be ground may remain an extended length of time in the mill, excluding any short cuts to the outlet. As a natural consequence, the quotient of the lateral length “L” divided by the diameter “D” of the vessel (L/D) is in the 3 to 4 range, as seen from FIG. 1 of the publication. This provides some disadvantages: if the material to be ground is high in viscosity, or if the bead-like grinding media is of relatively small sized pieces, the grinding media is apt to be driven toward the outlet, thereby preventing the even application of the crushing power to the “to be ground” material while being rubbed, and abnormal heat generation or wearing is caused. Sometimes, the mill may be overloaded in operation until it stalls. This problem grows noticeable more and more as the bead-like grinding media reduces less and less in particle size.
In the hope of preventing appearance of short cuts and making full use of the rotary energy it has been proposed that an annular space around the grinding vessel is used as the grinding chamber, but the problem of the grinding media being localized remains unsolved.